Method and apparatus for accessing old network through temporary id of evloved network

ABSTRACT

A method is provided for accessing a legacy network, such as a 2G/3G network, through a temporary ID of an evolved network. User Equipment (UE) adds Mobility Management Entity (MME) information for uniquely identifying an MME accessed by the UE on an evolved network to an access message sent to an old network when the UE accesses the legacy network through a temporary ID of the evolved network. The legacy network selects a corresponding Serving GPRS Support Node (SGSN) for the UE according to the access message. After the MME information for uniquely identifying an MME is added to the access message sent to the old network, the legacy network can determine and find the MME that is accessed by the UE in the evolved network without changing a Radio Network Controller (RNC) and an SGSN on the legacy network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/649,942,filed on Dec. 30, 2009, which is a continuation of InternationalApplication No. PCT/CN2008/072864, filed on Oct. 28, 2008. TheInternational Application claims priority to Chinese Patent ApplicationNo. 200710166066.8, filed on Nov. 1, 2007 and Chinese Patent ApplicationNo. 200810091433.7, filed on Apr. 11, 2008, all of which are herebyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to mobile communication technologies, andin particular, to a method and apparatus for accessing legacy (i.e.,pre-existing technology, such as 2G/3G) wireless networks through atemporary ID of an evolved network.

BACKGROUND OF THE INVENTION

A Universal Mobile Telecommunication System (UMTS) is a 3G wirelesscommunication network standard defined by the Third GenerationPartnership Project (3GPP). A UMTS network consists of a Core Network(CN) and an Access Network (AN). The CN consists of a Circuit Switched(CS) domain and a Packet Switched (PS) domain. The CS domain providesCS-based services such as voice services. The PS domain providesPS-based services such as Internet access. A terminal used by a mobilesubscriber is a User Equipment (UE).

In a prior art UMTS network, the CN generally includes a PS domain and aCS domain. The PS domain in the CN generally includes a Serving GPRSSupport Node (SGSN), a Gateway GPRS Support Node (GGSN), and a HomeLocation Register (HLR). The CS domain on the CN generally includes aMobile Switching Center (MSC), a Visitor Location Register (VLR), and aGateway Mobile Switching Center (GMSC). The AN generally includes aRadio Network Controller (RNC) and a NodeB. Each RNC is connected toseveral NodeBs. Each SGSN is connected to several RNCs. An Iu interfaceis a key interface between the AN and the CN. Management and control ofradio resources are isolated on the AN by the Iu interface, and thus theCN focuses solely on the service provision.

On a traditional network such as an R99 system, one RNC is connected toonly one CN node. For example, one RNC is connected to only one SGSN.Thus, problems such as single point failure exist. If an SGSN is down, aUE in the service area of the SGSN cannot access the network, and thuscannot perform communications. To rectify the foregoing defect, aconcept of “flex” is introduced. When a many-to-many relationshipbetween AN devices and CN devices exists on an Iu interface, the Iuinterface is called Iu-flex. In FIG. 1, there are Areas 1 to 8 governedrespectively by 8 RAN nodes. A configuration of a “pool area” in theprior art is shown herein. In FIG. 1, a Radio Access Network (RAN) node(for example: an RAN node may be an RNC in legacy wireless network suchas an R99 system) is connected to multiple SGSNs or multiple MSCs, andan SGSN or an MSC is connected to multiple RAN nodes. These SGSNs form apool, and the areas governed by the RAN nodes which the SGSNs connectare called pool area (for example: CS pool-area 1 including Areas 1, 2,5 and 6, CS pool-area 2 including Areas 2, 3, 6 and 7, PS pool-area 1including Areas 1 and 5, and PS pool-area 2 including Areas 2, 3, 6 and7). In a pool, multiple CN nodes such as SGSNs are connected to allRadio Access Network (RAN) nodes (such as RNCs) in the pool (forexample: SGSNs 3 to 5 are connected to each RAN node in PS pool-area 2and SGSNs 1 to 2 are connected to each RAN node in PS pool-area 1),which is different from a traditional mode where one AN node isconnected to only one CN node. When a UE enters a pool area initially,an RAN node can select one CN node according to load sharing principles.Thus, when the UE moves or accesses the network in the pool, the UE isalways anchored at the selected CN node. Therefore, single point failureand frequent relocation of CN nodes can be prevented because the UE doesnot need to change the CN node in the pool.

In the prior art, a network allocates a Temporary Mobile StationIdentity (TMSI, which is allocated by an MSC in a CS domain) or a P-TMSI(which is allocated by an SGSN in a PS domain) to a UE after the UE isattached to the network.

FIG. 2 shows a network with TMSI/P-TMSI design in a prior art. Thenetwork includes four pools (Pools 1 to 4) whose areas have overlappedparts. Each pool includes five CN devices, which are differentiated withdifferent Network Resource Identifiers (NRIs) (Pool 1 with NRIs 1 to 5,Pool 2 with NRIs 16 to 20, Pool 3 with NRIs 11 to 15, Pool 4 with NRIs 6to 10). A Non Access Stratum (NAS) Node Selection Function (NNSF) andthe TMSI uniqueness of a UE in a paging area are not affected, andtherefore, duplicate NRIs can be used in non-adjacent pool areas (forexample, NRI 11 can be reused in a pool area not adjacent to arearelated to pool 3, NRI 1 can be reused in a pool area not adjacent toarea related to pool 1). Assume that each CN device can attach a maximumof 1,000,000 subscribers, while the overlapped pool areas have12,000,000 subscribers, and other areas have fewer subscribers.

On the preceding network, 20 CN devices are sufficient to attach12,000,000 subscribers. An NRI may be set to 5 bits (2⁵=32, which can beused to identify 20 CN devices). The independently allocated ID of eachdevice is 21 bits (1000000=2²⁰, which can be used to identify 2,000,000subscribers), two bits are used to differentiate a PS domain from a CSdomain, and the remaining four (32−5−21−2=4) bits are used for restart.

FIG. 3 shows a structure of the flex on an SAE network in the prior art.In a pool (such as Mobility Management Entity (MME) pool 1 or MME pool2), multiple CN nodes such as MMEs (not shown in the figure) areconnected to all RAN nodes such as eNodeBs (ENBs) 1 to 4, which issimilar to the method of the prior art. When a UE initially enters thearea related to the pool, an RAN node can select one CN node accordingto load sharing principles. Thus, when the UE moves or accesses thenetwork in this pool, the UE is always anchored at the selected CN node.On the SAE network, pool areas may also be overlapped. In addition, theSAE network specifies that an MME pool area or an S-GW pool areaincludes a complete Tracking Area (TA, which is similar to a LocationArea (LA) or a Routing Area (RA) on a UMTS network).

Assume that a UE is allocated one TA once. When the UE enters MME pool 1(hereinafter referred to as MP1) for the first time, for example, whenthe UE enters ENB1, the UE selects one MME from MP1. When the UE movesfrom ENB1 to ENB2 and then to ENB3, the UE does not need to change theMME. When the UE moves to ENB4 that is not connected to the source MMEand belongs only to MP2, the UE may reselect an MME in MP2. In FIG. 3,ENB2 and ENB3 belong to two MME pools; that is, ENB2 and ENB3 areconnected to all MMEs in the two MME pools. Therefore, ENB2 and ENB3 areoverlapped parts of MP1 and MP2. The advantage of overlapping is asfollows: because ENB3 is connected to MP2, when the UE returns from ENB4to ENB3, the UE does not need to reselect an MME until the UE returns toENB1, thus preventing ping-pong MME relocation. If ENB3 is not connectedto MP2, ping-pong MME relocation occurs when the UE moves between ENB3and ENB4.

With respect to the TA concept, it should be noted that on an SAEnetwork, multiple TAs can be allocated to a UE, which is different fromthe practice in a UMTS network where only one LA or RA can be allocatedto a UE. In FIG. 3, if a UE is registered in a pool, and if the TA listincludes TA1 and TA2, the UE does not need to initiate an update whenmoving between ENB1 and ENB2. That is, the UE does not need to initiatean update when moving in the allocated TA list.

Currently, a TMSI problem about the SAE is as follows: the SAE needs tosupport multiple Radio Access Technologies (RATs), and therefore,several types of terminals access the network; to keep the systemcapacity, an SAE-TMSI (namely, SAE TMSI) may need to be extended tosupport more subscribers, to expand the capacity, and to simplify thenetwork.

When assessing the operation of prior art networks the inventoridentified at least the following problem: when a UE moves from an SAEnetwork to a legacy (such as 2G/3G) network, the legacy network cannotidentify the SAE-TMSI, and therefore, the newly selected SGSN cannotfind the originally-assigned MME on the SAE network. As a result, thenew SGSN cannot obtain the context of the UE. And, ongoing services forthe UE maybe interrupted.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a method and apparatus foraccessing a legacy network through a temporary ID of an evolved networkto rectify a technical defect in the prior art that a newly selectedSGSN cannot obtain the context of a UE because the new SGSN cannot findthe originally-assigned MME on an SAE network.

An embodiment of the invention provides a method for accessing a legacynetwork, such as a 2G/3G network. The method includes:

obtaining, by a User Equipment (UE), a temporary identity (ID) allocatedby a Mobility Management Entity (MME) in an evolved network, where thetemporary ID comprises MME information for identifying the MME;

adding, by the UE, the MME information from the temporary ID to a firstP-Temporary Mobile Station Identity (P-TMSI) in an access message;

sending, by the UE, the access message to a Radio Access Network (RAN)node in the 2G/3G network.

An embodiment of the invention further provides a UE, which includes areceiver, a transmitter and a Mobility Management Entity (MME)information adding module.

The receiver is configured to obtain a temporary identity (ID) allocatedby an MME in an evolved network, wherein the temporary ID comprises MMEinformation for identifying the MME; the MME information adding moduleis configured to add the MME information from the temporary ID to afirst P-Temporary Mobile Station Identity (P-TMSI) in an access message,when the UE accessing a 2G/3G network; and the transmitter is configuredto send the access message to a Radio Access Network (RAN) node in the2G/3G network.

An embodiment of the invention further provides a Radio Access Network(RAN) node in a 2G/3G network. The RAN node includes:

a receiver, configured to receive an access message sent by a UserEquipment (UE) that accesses the 2G/3G network, wherein the accessmessage comprises Mobility Management Entity (MME) informationidentifying an MME accessed by the UE in an evolved network, and the MMEinformation is added by the UE from a temporary ID allocated by the MMEto a first P-Temporary Mobile Station Identity (P-TMSI) in the accessmessage; and

a processor, configured to select a corresponding Serving GPRS SupportNode (SGSN) in the 2G/3G network for the UE according to the firstP-TMSI in the access message.

The technical solution in embodiments of the invention has the followingadvantage: when a UE moves from an evolved network to an old network,MME information for uniquely identifying an MME is added to an accessmessage sent to the old network; therefore, the old network candetermine and find the MME that is accessed by the UE in the evolvednetwork without changing the RNC and SGSN on the old network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration of a pool area in the prior art;

FIG. 2 shows a network with TMSI/P-TMSI design in the prior art;

FIG. 3 shows a structure of the flex on an SAE network in the prior art;

FIG. 4 shows a structure of a TMSI/P-TMSI in the prior art;

FIG. 5 is a flowchart of a method for accessing a legacy network througha temporary ID of an evolved network in a first embodiment of theinvention;

FIG. 6 shows how to select a combined MME/SGSN in an embodiment of theinvention;

FIG. 7 shows how to add a Pool-id and an MME-id to Routing Area Update(RAU) information and P-TMSI information in an embodiment of theinvention;

FIG. 8 shows how to add a TAI, a Pool-id, and an MME-id to RAUinformation and P-TMSI information in an embodiment of the invention;

FIG. 9 shows how to occupy a Radio Resource Control (RRC) message in anembodiment of the invention;

FIG. 10 shows how to occupy a P-TMSI Signature in an embodiment of theinvention;

FIG. 11 shows a structure of a UE in an embodiment of the invention; and

FIG. 12 shows structures of a UE and a RAN node in an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are hereinafter described in detail withreference to the accompanying drawings, in which corresponding referencecharacters represent corresponding elements.

A TMSI/P-TMSI is introduced to prevent a permanent ID, namely, anInternational Mobile Subscriber Identity (IMSI), from being exposed onan air interface and thus being tracked, so as to protect subscriberprivacy. The TMSI is unique in an LA or RA of a UE (LA in a CS domainand RA in a PS domain, it being understood that one LA may includeseveral RAs). Thus, when the UE accesses a network, the TMSI/P-TMSI istaken as the user ID. In addition, the TMSI/P-TMSI has the followingfunction: when the UE accesses a new CN node and the flex concept is notintroduced, the new CN node searches old nodes for the context of the UEthrough a Location Area Identity (LAI) or Routing Area Identity (RAI). Amany-to-many relationship between AN devices and CN devices exists onthe flex, and therefore, it is insufficient to search old nodesaccording to the LAI or RAI. Old nodes may be determined according tothe LAI/RAI and the TMSI/P-TMSI.

The TMSI/P-TMSI has another function, that is, an RAN can find a nodewhere the UE is registered according to the TMSI or P-TMSI informationin the case of the flex. The specific method is as follows: theconfigurable bits 0-10 available in the TMSI or P-TMSI are used toidentify a Network Resource Identifier (NRI) which is used todifferentiate different CN nodes in a pool. Thus, when the UE accessesthe pool for the first time, an RAN node selects a proper CN node forthe UE according to the NNSF and load sharing principles. When the UEmoves in the pool, the UE does not change the selected node. Theprinciple is as follows: the CN node allocates one TMSI or P-TMSI to theUE and the TMSI or P-TMSI carries an NRI that represents the CN node.When the UE accesses the pool, an Initial Direct Transfer message sentby the UE carries the TMSI or P-TMSI. According to the NRI in the TMSIor P-TMSI, the RAN node finds and selects the CN node where the UE isoriginally registered. Thus, when the UE moves in the pool, the UE doesnot change the CN node. When the UE moves out of the pool, the RAN nodereselects a new CN node because it cannot find the corresponding NRI.Subsequently, when the UE moves in the new pool, the UE still does notchange the new CN node.

A TMSI or P-TMSI in the prior art consists of 32 bits, namely, severalbits used to differentiate a PS domain from a CS domain (two bitsgenerally), configurable bits 0-10 used to identify an NRI (Bit 0indicates no flex), several bits used as a restart ID, and several otherbits. These bits may be allocated according to the network deploymentsituation. FIG. 4 shows a structure of the TMSI or P-TMSI in a priorart. Two bits are used to differentiate the TMSI from the P-TMSI. A5-bit restart ID is used to prevent that the allocated TMSI isreallocated because of node restart; the 5-bit restart ID may beassociated with the time to prevent duplicate TMSIs from beingallocated. Seven bits are used to identify an NRI. The remaining 18 bitsmay be used as an ID allocated to the UE by each CN device.

In a prior art, an RNC selects a new SGSN for a UE, and the new SGSNrequests the context of the UE from the former (i.e., legacy network)SGSN according to the RAI+P-TMSI. The process is as follows: The UEinitiates an RAU process and sends an RRC message (Initial DirectTransfer message) to the RNC. The message includes an RAU Requestcarrying the former RAI and former P-TMSI, where the RAU Request islocated in the Non Access Stratum (NAS) information element (IE) of theRRC message. The RNC can see only the RRC message but does not parse theNAS message. According to the NRI information in the P-TMSI fieldcarried on an Intra Domain NAS Node Selector Network Element (NE) of theRRC message, the RNC selects the corresponding SGSN, establishes aconnection with the SGSN, and forwards the NAS message to the SGSN. Ifthe RNC does not find the corresponding SGSN, it selects a new SGSN (inthis process, the RNC selects a new SGSN).

According to the RAI and P-TMSI information carried in the received RAURequest, the new SGSN queries the address of the original SGSN in itsconfiguration or domain name server (DNS). Then, the new SGSN sends aSGSN Context Request carrying the old RAI and old P-TMSI to the old SGSNto request the context of the UE. According to the old RAI and oldP-TMSI information, the old SGSN finds the corresponding UE and returnsthe context of the UE to the new SGSN. After executing other processessuch as updating a location to a Home Subscriber Server (HSS), the newSGSN allocates a new RAI and P-TMSI to the UE.

When the UE moves from an SAE network to a legacy network, the legacynetwork cannot identify the SAE-TMSI, and therefore, the newly selectedSGSN cannot locate the former MME on the SAE network. As a result, thecontext of the UE cannot be obtained. In embodiments of the invention,when the UE moves from an evolved network such as an SAE network to alegacy network such as a 2G/3G network, the UE notifies the MMEinformation of the MME where the UE is registered on the evolved networkto the old network by sending an access message such as RAU or Attach tothe old network. The MME information can uniquely identify the MMEaccessed by the UE in the evolved network. Thus, after the old networkselects a new SGSN for the UE, the new SGSN determines and finds the MMEaccording to the MME information in the access message, and then sends aContext Request or Identification Request to the MME to obtain thecontext (in the RAU process) or IMSI information (in the Attach process)of the UE. If a combined MME/SGSN exists, when the UE is registered withthe combined node, and the UE moves from an SAE system to a 2G/3G poolor from a 2G/3G pool to an SAE system, it is better to select the oldcombined node if possible to prevent context transfer. Embodiments ofthe invention can also implement the function.

According to different cases of MME information, embodiments of theinvention provide different methods for adding the MME information to anaccess message. For example, if one MME corresponds to one MME-id on anevolved network, the MME-id can uniquely identify the MME; therefore,only MME-id being added to the P-TMSI or RAI information in the accessmessage may works. If a pool exists on the evolved network, an MME has aunique MME-id in the pool; therefore, the Pool-id+MME-id should be usedto determine the MME. If Pool-ids on different Public Land MobileNetworks (PLMNs) are duplicate, the PLMN-id+Pool-id+MME-id can uniquelydetermine the MME. In addition, the TAI+Pool-id+MME-id orPLMN-id+TAI+Pool-id+MME-id may also uniquely determine the MME. The poolis an area where a UE moves and the serving MME does not need to bechanged. The following describes the preceding cases provided inembodiments of the invention in detail.

In embodiments of the invention, it is assumed that the UE accesses alegacy network (such as a 2G/3G network) through a temporary user ID(SAE-TMSI) of an evolved network. In some embodiments of the invention,a structure of the SAE-TMSI is provided, which includes the Pool-id,MME-id, UE temporary id, optional restart ID, and optional other IDs(the PLMN-id may be or may not be taken as one part of the SAE-TMSI).That is, the SAE-TMSI contains at least the Pool-id, MME-id, and UE-id.The Pool-id indicates the ID of a pool or area where the MME is located.The MME-id indicates the ID of an MME in the pool. The UE-id indicatesan ID that uniquely identifies a UE in an MME. That is, the MME-id ofthe entire network is indicated by the Pool-id+MME-id.

The Pool-id may be unique or duplicate on the entire PLMN. The IDs ofnon-adjacent pools may be the same. Thus, if Pool-ids are duplicate, theSAE-TMSI and TAI information should be used together to identify an MME.The MME-id is a unique ID in the pool. The UE temporary id is an ID thatmay be allocated to the UE by each MME. A restart-id may also be addedto the SAE-TMSI. The function of the restart-id is similar to therestart-id in a P-TMSI in a prior art; that is, the function is toprevent the same SAE-TMSIs from being allocated in the same MME. Inaddition, an ID used to differentiate a UMTS/SAE network or other IDsused to differentiate networks can be added to the SAE-TMSI.

The foregoing SAE-TMSI structure is a solution provided in embodimentsof the invention, where an RNC and an SGSN on a prior network do notneed to be changed. A benefit of use of the invention is that legacynetworks can uniquely determine and identify an MME that is accessed bythe UE on an evolved network, and thus obtain the context of the UEafter the UE accesses the old network through the SAE-TMSI. When thefollowing requirements are met, any change of the SAE-TMSI structureshould be covered in the scope of protection of the invention.Embodiments of the invention take into account the followingconsiderations for the SAE-TMSI:

1. The SAE-TMSI is secure and not easy to track.

2. An RAN selects an old CN node.

3. If a CN node changes, a new CN node finds the old CN node to obtainthe context or ID of a UE through an LA or through an LA and a TMSI.

4. The corresponding UE is found in an old CN node when the SAE-TMSIfunctions as an index.

Obviously, the IDs that meet the preceding requirements for the SAE-TMSIare still in the SAE-TMSI range although their names are different.

FIG. 5 is a flowchart of a method for accessing an old network through atemporary ID of an evolved network in the first embodiment of theinvention. The flowchart includes the following steps:

Step S701: When a UE initially accesses an evolved network, the evolvednetwork selects the corresponding MME for the UE. For example, theevolved network selects the MME according to load sharing principles.After the UE is registered with the MME, the MME allocates an SAE-TMSIfor the UE.

Step S702: If the UE moves from the evolved network to a legacy network(such as a 2G/3G network) and accesses the legacy network through theSAE-TMSI allocated by the evolved network, MME information for uniquelyidentifying the MME accessed by the UE in the evolved network is addedto an access message sent from the UE to the legacy network. Because theSAE network configurations are different, MME information has severalmodes. The following describes different modes of carrying MMEinformation.

Mode 1: On the evolved network, the MME has a unique MME-id, based onwhich the legacy network can determine and find the MME that is accessedby the UE in the evolved network. In an embodiment of the invention, theUE adds the MME-id to the RAI information and/or P-TMSI informationcarried in the access message. In another embodiment of the invention,the UE adds the MME-id to the P-TMSI information. The UE may also addthe MME-id to the P-TMSI Signature of the access message.

On the evolved network, some of the functionality of the MME and SGSNmay be combined. In this case, the legacy network may reselect anotherSGSN rather than the combined MME/SGSN for the UE when the UE moves fromthe evolved network to the legacy network (2G/3G network) in the samepool as the evolved network, and accesses the legacy network. Thiscauses unnecessary node reselection and context transfer. Therefore, theNRI and the MME-id of the combined MME/SGSN may be the same. Inaddition, when the UE moves from the evolved network to the old network,the NRI information in the P-TMSI information carried in the accessmessage is set as the MME-id. Thus, when the old network selects an SGSNfor the UE, it selects the old combined MME/SGSN, thus avoiding nodereselection. FIG. 6 shows a configuration of combined MME/SGSN in anembodiment of the invention. Blocks 1 to 7 indicate SGSN or MME with NRIor MME-id arranged from 1 to 7. Assume that blocks 4 indicates acombined MME/SGSN whose NRI and MME-id are equal to 4, blocks 6indicates a combined MME/SGSN whose NRI and MME-id are equal to 6, andthat blocks 1, 2, and 3 indicate a pure MME or SGSN whose NRI or MME-idis equal to 1, 2, and 3 respectively. Blocks 4, 5, 6 and 7 belong topool 1 with Pool-id 1, blocks 1, 2, 3 and 4 belong to pool 2 withPool-id 2 and blocks 5, 6 and 7 belong to pool 3 with Pool-id 3.According to a embodiment of the invention, when the UE is registeredwith node 6 (the combined MME/SGSN indicated by block 6) in pool 1, andis still in pool 1 when it moves from an SAE network to a 2G/3G network,the UE initiates access in the 2G/3G network, and uses an access messageof the 2G/3G network and places the MME-id part of the SAE-TMSI to theNRI part of the P-TMSI. Thus, upon reception of the message, a BSC or anRNC forwards the message to node 6 to establish a connection with node 6through the information of NRI=6, and therefore, the combined SGSN/MMEindicated by block 6 can still be selected, and the node does notchange. When moving to a 2G/3G network in pool 2, the UE initiates anRAU process. A BSC or an RNC receives an Initial Direct Transfermessage. According to the information of NRI=6, the BSC or RNC cannotfind the corresponding node, and therefore, it initiates node selectionand selects a new SGSN with NRI=2. After receiving the RAU Request, theSGSN queries information in the configuration or DNS according to theRAI and P-TMSI information. If the DNS is upgraded or the configurationis changed, the corresponding old MME address may be found according tothe MME-id (or Pool-id+MME-id or PLMN-id+Pool-id+MME-id). The new SGSNsends a Context Request carrying RAI and P-TMSI information to the MME.According to the implicit SAE-TMSI information in the RAI and P-TMSI,the old MME finds the corresponding UE and returns the context of the UEto the new SGSN.

According to the foregoing descriptions, placing the MME-id part of theSAE-TMSI to the NRI part of the P-TMSI is one solution of the embodimentof the invention. Thus, unnecessary node reselection can be avoidedefficiently.

In addition, if the UE moves from the 2G/3G network (old network) to theSAE network (evolved network), the UE accesses the SAE network throughthe P-TMSI. An ENB (RAN node of the evolved network) selects the MMEwith MME-id=NRI. For example, if the UE moves from the SAE network tothe 2G/3G network, and the UE has been registered with the combined node6 in pool 1, the accessed 2G/3G network still belongs to pool 1. Then,the ENB still selects the old combined MME/SGSN indicated by block 6. Ifthe ENB cannot find the corresponding MME, it triggers a new MMEselection process and selects a new MME for the UE.

Mode 2: On the evolved network, the MME-id is unique in the same pool,but duplicate IDs may exist on the entire evolved network. The Pool-idis unique on the entire evolved network (PLMN). Therefore, thePool-id+MME-id can uniquely identify the MME accessed by the UE in theevolved network. In an embodiment of the invention, the SAE-TMSIcontains the unique Pool-id on the PLMN, unique MME-id in a pool, andunique UE-id (and other possible fields) in an MME. That is, the MME canbe identified by the unique Pool-id on the PLMN and the unique MME-id inthe pool. The UE adds the Pool-id and the MME-id to the RAI and/orP-TMSI information carried in the access message; that is, the Pool-idand the MME-id are mapped to the RAI and/or P-TMSI field. In anembodiment of the invention, the Pool-id and the MME-id are added to theRAI information and the P-TMSI information. The NRI information of theP-TMSI information in the access message is set as the MME-id, and thePool-id is added to other information except the NRI (such as the otherpart of the P-TMSI or the LAC and/or RAC part of the RAI, or other partof the RAI except the Mobile Country Code (MCC) and the Mobile NetworkCode (MNC) and other part of the P-TMSI except the NRI); otherinformation includes P-TMSI information, RAI information, or P-TMSISignature. The RAI or P-TMSI information may also carry other IEs of theSAE-TMSI. FIG. 7 shows how to add the Pool-id and the MME-id to RAIinformation and P-TMSI information in an embodiment of the invention.For example, in FIG. 7, the Pool-id is added to the RAI. The Pool-id mayalso occupy the P-TMSI or a part of RAI and a part of P-TMSI. To avoidunnecessary node reselection when a combined MME/SGSN exists, in anotherembodiment of the invention, the MME-id part of the SAE-TMSI is placedto the NRI part of the P-TMSI (the NRI and the MME-id of the combinedMME/SGSN may be the same). If the SAE-TMSI is longer than the P-TMSI(for example, the length is 40 bits or 56 bits), the other part mayoccupy certain bits of the RAI and/or certain bits of the P-TMSISignature (not shown in FIG. 7). In an embodiment of the invention, onlycertain bits of the RAI are occupied to prevent the SGSN and the RNCfrom being changed.

The Pool-id and other part of the SAE-TMSI may be placed to the P-TMSIinformation except the NRI. If the SAE-TMSI is longer than the P-TMSI,the LAC and/or RAC part of the RAI can also be occupied. Thus, thelength of the SAE-TMSI can be extended to a maximum of 56 bits.Therefore, the mode does not limit the number of MMEs on the PLMN. Thatis, the number of MMEs depends on the Pool-id+MME-id. The maximum numberof MMEs in a pool is 1024 (2¹⁰ because the maximum MME-id is the lengthof the NRI, namely, 10 bits). If no interaction with the 2G/3G networkis required, the number of MMEs in a pool is not limited by the lengthof the NRI.

Similarly, if the UE moves from the 2G/3G network to the SAE network,the UE accesses the SAE network through the P-TMSI. In this case, theRAN node such as the ENB of the evolved network tries to select the MMEwith MME-id=NRI. For example, if the UE moves from the SAE network tothe 2G/3G network, and the UE has been registered with the combined node6 in pool 1, the accessed 2G/3G network still belongs to pool 1. Then,the ENB still selects the old (i.e., pre-existing) combined MME/SGSN(6). If the ENB cannot find the corresponding MME, it triggers a new MMEselection process and selects a new MME for the UE.

Therefore, an embodiment of the invention provides an optimized mode: inthe case of a pure SAE network, Pool-IDs may not be duplicate on thePLMN and MME-ids may not be duplicate in a pool; in the case of a hybridnetwork of an SAE network and a 2G/3G network, the NRI of the SGSN ofthe combined MME/SGSN may be equal to the MME-ID of the MME of thecombined MME/SGSN, and the MME-ID of a pure MME should not be the sameas the NRI of a pure SGSN in adjacent pools.

It should be noted that after the UE accesses the 2G/3G network throughthe SAE-TMSI and selects a new SGSN, the SGSN may search the DNS orconfiguration for the old (i.e., pre-existing) node according to theP-TMSI and the RAI (SAE-TMSI information actually from the SGSN point ofview). If the DNS or configuration is upgraded, the old MME address canbe uniquely determined and found according to the PLMN-id (MCC+MNC inthe RAI) in the SAE-TMSI information, Pool-ID, and MME-id. The SGSNsends a Context Request or Identification Request message carrying theP-TMSI and the old RAI (including the SAE-TMSI actually from the SGSNpoint of view). After receiving the message, the MME finds the UEaccording to the SAE-TMSI and returns the context of the UE.

Mode 3: In this mode, the PLMN-id belongs to a part of the SAE-TMSI. Onthe evolved network, the MME-id is unique in a pool, but duplicate IDsmay exist on the entire evolved network. A Pool-id is not unique ondifferent PLMNs. Therefore, the PLMN-id (MCC+MNC) should be used withthe Pool-id+MME-id to identify the MME accessed by the UE in the evolvednetwork. That is, the MME is uniquely identified by thePLMN-id+Pool-id+MME-id. As shown in FIG. 7, the NRI information of theP-TMSI information in the access message is set as the MME-id. The RAI,P-TMSI, and P-TMSI Signature can carry the PLMN-id and the Pool-id. Forexample, the PLMN-id (MCC and MNC in FIG. 7) is added to the RAIinformation, and the Pool-id is added to the other part of the P-TMSIinformation except the NRI; the PLMN-id and the Pool-id can be added tothe RAI information or P-TMSI Signature information (Not shown in FIG.7). In an embodiment of the invention, a specific mode is to set the NRIinformation as the MME-id. The RAI, P-TMSI Signature, and P-TMSI canseparately carry or together carry the PLMN-id and the Pool-id.

To avoid unnecessary node reselection when a combined MME/SGSN exists,another embodiment of the invention places the MME-id part of theSAE-TMSI to the NRI part of the P-TMSI (the NRI and the MME-id of thecombined MME/SGSN may be the same).

Mode 4: When the Pool-id and the MME-id are not unique on the PLMN, forexample, Pool-ids on the PLMN may be duplicate (Pool-ids of adjacentpools should be different), the old MME may be determined by combiningthe Pool-id and the MME-id with the TAI; that is, the MME is uniquelyidentified by the TAI+Pool-id+MME-id. FIG. 8 shows how to add a TAI, aPool-id, and an MME-id to RAU information and P-TMSI information in anembodiment of the invention. The UE may provide the TAI information,which is shown as TAC in FIG. 8, to the old network. Because theSAE-TMSI may be extended, the SAE-TMSI+TAI may occupy the position ofthe old RAI+P-TMSI. If the TAC occupies 20 bits, four bits may beprovided for the SAE-TMSI. Thus, the SAE-TMSI has 36 bits. (The P-TMSIfield is 32 bits in length, and the LAC+RAC is 24 bits in length.) Ifthe UE accesses a 2G/3G network through the SAE-TMSI, the MME selects aproper SGSN at the NRI position, and the SGSN searches the updated DNSor configuration for the old MME and finds the old MME through theTAI+Pool-id+MME-id. The Pool-id may be combined with the MME-id into oneMME-id. In an embodiment of the invention, the NRI information in theP-TMSI information of the access message is set as the MME-id, and theTAI and the Pool-id are added to the RAI information. To avoidunnecessary node reselection when a combined MME/SGSN exists, anotherembodiment of the invention places the MME-id part of the SAE-TMSI tothe NRI part of the P-TMSI (the NRI and the MME-id of the combinedMME/SGSN may be the same).

Step S703: The legacy network selects the corresponding SGSN for the UEaccording to the access message. According to the NRI of the P-TMSIinformation carried in the received access message, the legacy networkselects the corresponding SGSN. If no corresponding SGSN exists, the oldnetwork selects a new SGSN. If the UE accesses the new SGSN, the UEsends the RAI IE and P-TMSI information carried in the NAS message tothe new SGSN.

Step S704: If the selected SGSN is a new node, the selected SGSNdetermines an MME according to the MME information carried in the accessmessage and requests the UE information from the MME. Specifically, theconfiguration information or DNS of the SGSN is upgraded, and the SGSNsearches the configuration information or DNS for the MME according tothe MME information. For example, the corresponding address of the oldMME can be found through the MME-id (or Pool-id+MME-id orPLMN-id+Pool-id+MME-id), and the new SGSN sends a Context Requestcarrying the RAI and P-TMSI information to the MME; the old MME findsthe corresponding UE according to the implicit SAE-TMSI information inthe RAI and P-TMSI and returns the context of the UE to the new SGSN.

Mode 5: According to the foregoing embodiments, an embodiment of theinvention provides a embodiment of carrying SAE-TMSI information isprovided. The mode for a UE to access an old (i.e., legacy) network suchas a 2G/3G network in the prior art is as follows: P-TMSI information isplaced in a received RRC message (Initial Direct Transfer message), andan RAN node such as an RNC or a BSC finds the corresponding SGSNaccording to the NRI information in the P-TMSI information; in addition,the NAS message in the RRC message carries P-TMSI information, that is,two P-TMSIs; the P-TMSI (and the old RAI, as well as the possible P-TMSISignature) in the NAS message is transferred to an SGSN, and the RANnode such as the RNC or BSC does not parse the P-TMSI information in theNAS message. Therefore, the method of this mode is as follows: only theP-TMSI in the RRC message is changed; the NRI information is set as theMME-id; and the information of the P-TMSI in the RRC message except theNRI information may not be processed, for example, may be set to zero.With regard to the P-TMSI, RAI, and P-TMSI Signature in the NAS message,the MME information is placed to the P-TMSI and/or the RAI only, andthus the old MME address can be found in the DNS or SGSN configuration.The MME-id does not need to be placed in the NRI information of theP-TMSI information in the NAS message. FIG. 9 shows an embodiment ofusing RRC message when a UE accesses an old 2G/3G network through theSAE-TMSI. As shown in FIG. 9, the NRI information of the P-TMSIinformation in an RRC message is set to MME information, such as theMME-id. In the NAS message, the old MME should be found according to theold RAI and the P-TMSI only. For example, the Pool-id and the MME-id areset in the old RAI, the UE-id and other bytes are placed in the P-TMSI.In another embodiment of the invention the MME-id does not need to beplaced in the NRI information of the P-TMSI information in the NASmessage. Other implementation modes may also be adopted. For example,the global MME-id is placed to the old RAI, or the TAI is placed to theP-TMSI, or a duplicate MME-id is placed to the old RAI, and even theP-TMSI Signature can be used (for example, the global MME-id is placedto the old RAI and the P-TMSI, and the UE-id is placed to the P-TMSISignature). Thus, the length of the SAE-TMSI can be further extended.The MME information such as the Pool-id, MME-id, and TAI are placed tothe old RAI and/or P-TMSI, thus reducing the configurations of DNSs orSGSNs.

According to the DNS or configuration, the pre-existing MME/SGSN can beidentified. It is possible that the identified pre-existing MME/SGSN isnot the actual previously-assigned MME/SGSN of the UE. For example, ifthe UE accesses an SGSN that is not configured with the flex, of anotherPLMN, and the SGSN searches for the address of the default MME/SGSNaccording to only the RAI information, the new SGSN sends a ContextRequest or Identity Request message carrying RAI and P-TMSI informationto the MME/SGSN. After receiving the message, the default MME/SGSNidentifies the actual MME/SGSN according to the RAI and P-TMSIinformation, and then forwards the message to the actual MME/SGSN.

If the UE accesses the SAE network again through the allocated SAE-TMSI,an eNB can select the pre-existing MME according to the MME-id orPool-id of the SAE-TMSI and the MME-id. If there is no correspondingMME, the eNB selects a new MME. The new MME identifies the pre-existingMME according to the Pool-id and the MME-id in the SAE-TMSI. Thepre-existing MME identifies the UE according to the SAE-TMSI and returnsthe context of the UE to the new MME.

Mode 6: According to the foregoing embodiments, an embodiment of theinvention further provides another embodiment of carrying SAE-TMSIinformation is provided. In the foregoing modes, the PLMN-id, TAI, andPool-id (or MME-id) may be added to the access message by occupying theP-TMSI Signature. In this mode, the method for occupying the P-TMSISignature is primarily described, but it does not means that this modecan be implemented by occupying the P-TMSI Signature only. Theembodiment can be used with other information, such as RAI informationand/or P-TMSI information.

The P-TMSI Signature is used to ensure the security. When a UE accessesa 2G/3G network, the UE can carry the pre-existing RAI, P-TMSI, andP-TMSI Signature to the access network. If an SGSN changes, the new SGSNuses the pre-existing RAI and the P-TMSI to query the address of thepreviously-assigned SGSN. Then, the new SGSN sends an IdentificationRequest and a Context Request that may carry the optional parameterP-TMSI Signature besides the old RAI and P-TMSI, to the pre-existingSGSN. Therefore, when the UE accesses the legacy network through theSAE-TMSI, the SAE-TMSI may occupy the P-TMSI Signature when the new SGSNfinds the address of the old MME according to the old RAI and theP-TMSI. FIG. 10 shows how to occupy the P-TMSI Signature in anembodiment of the invention. Assume that the SAE-TMSI in the embodimentconsists of at least the Pool-id (the Pool-id is unique on the entirePLMN), MME-id, and UE-id. The Pool-id and the MME-id may be placed tothe pre-existing RAI and/or P-TMSI information. The MME-id may still beplaced in the NRI. Other fields occupy not only the old RAI and P-TMSIinformation but also certain bits of the P-TMSI Signature. For example,the P-TMSI Signature has 24 bits, where ten bits are used for SAEsecurity, and the remaining part may occupy certain bits (informationnecessary for querying the address of the old MME, such as the Pool-idand the MME-id cannot be placed in these bits) such as six bitsaccording to the configuration length of the SAE-TMSI. Thus, after theUE accesses a new SGSN, the new SGSN queries the address of the old MMEaccording to the old RAI and the P-TMSI. The new SGSN can find theaddress of the old MME according to the PLMN-id+Pool-id+MME-id if theconfiguration of the DNS or SGSN is upgraded. The new SGSN sends aContext Request carrying the old RAI, P-TMSI, P-TMSI Signature to theaddress of the old MME. The MME identifies the corresponding UE throughthe SAE-TMSI formed by these parameters, and returns the context of theUE. In another embodiment, such as the Mode 5 discussed above, theMME-id of the P-TMSI information in a NAS message does not need to beplaced in the NRI information.

The Pool-id may not be unique, and in some aspects of the invention, isnot even required, and the corresponding MME is determined by acombination of the TAI information and the MME-id. For example, the UEaccesses a 2G/3G network through the SAE-TMSI and the old TAI. TheSAE-TMSI includes an MME-id (The MME-id may be unique on the entire PLMNor unique in a pool. In the embodiment, assume that the MME-id is uniquein a pool). The UE places the TAI information to the old RAI informationand places the MME-id to the NRI information. The remaining informationoccupies the position of other remaining information of the old RAI andthe P-TMSI, and even occupies the P-TMSI Signature that does notparticipate in query of the old MME. If the old MME can be queriedaccording to the old RAI and the P-TMSI (the TAI and the MME-idactually), the other part of the SAE-TMSI that does not participate inquery of the old MME may occupy the P-TMSI Signature. The new SGSN sendsa request to the found MME address. The MME finds the UE according to aregrouped SAE-TMSI according to the parameters that are sent and returnsa response.

The length of the SAE-TMSI may reach 80 bits by occupying the P-TMSISignature. (Note that the information such as the MME-id, Pool-id, andTAI information used to query the MME cannot be placed to the P-TMSISignature.)

In the foregoing embodiments, when a UE moves from an evolved network toan old network, MME information for uniquely identifying an MME is addedto an access message sent to the old network; therefore, the old networkcan determine and find the MME that is accessed by the UE in the evolvednetwork without changing an RNC and an SGSN on the old network.Especially, when a combined MME/SGSN exists, unnecessary modereselection can be avoided efficiently by setting the MME-id of the MMEin the combined node to be the same as the NRI of the SGSN. (Aconfiguration method can also be used, and therefore, the MME-id may bedifferent from the NRI.)

FIG. 11 shows a structure of a UE 100 in an embodiment of the invention.The UE 100 includes an access message sending module 110 and an MMEinformation adding module 120. The access message sending module 110 isadapted to send an access message to an old network when the UE 100moves from an evolved network to the old network. The MME informationadding module 120 is adapted to add MME information for uniquelyidentifying an MME accessed by the UE 100 on the evolved network to theaccess message sent by the access message sending module 110.

The MME information adding module 120 includes a P-TMSI informationadding submodule 121, which is adapted to add the MME-id in an SAE-TMSIto the P-TMSI information of the access message.

The MME information adding module 120 includes an RAI information addingsubmodule 122, which is adapted to add the Pool-id, and PLMN-id or TAIor PLMN in the SAE-TMSI to the RAI information of the access message.

The MME information adding module 120 further includes an NRI settingsubmodule 123, which is adapted to set the NRI information in the P-TMSIinformation of the access message as the MME-id, thus avoidingunnecessary node reselection when a combined MME/SGSN exists.

The UE 100 further includes a P-TMSI Signature adding module 130, whichis adapted to add other information in the SAE-TMSI except the MMEinformation to the P-TMSI Signature.

FIG. 12 shows structures of a UE 200 and a RAN node 300 in an embodimentof the invention.

The UE 200 includes a receiver 201, a transmitter 203 and a MMEinformation adding module 202.

The receiver 201 is configured to obtain a temporary identity (ID)allocated by an MME in an evolved network, wherein the temporary IDcomprises MME information for identifying the MME.

The MME information adding module 202 is configured to add the MMEinformation from the temporary ID to a first P-TMSI in an accessmessage, when the UE accessing a 2G/3G network.

The transmitter is configured to send the access message to the RAN node(such as the RAN node 300) in the 2G/3G network.

The RAN node 300 includes a receiver 301 and a processor 302.

The receiver 301 is configured to receive an access message sent by a UE(such as the UE 200) that accesses the 2G/3G network, wherein the accessmessage comprises MME information identifying an MME accessed by the UEin an evolved network, and the MME information is added by the UE from atemporary ID allocated by the MME to a first P-TMSI in the accessmessage; and

The processor 302 is configured to select a corresponding SGSN in the2G/3G network for the UE according to the first P-TMSI in the accessmessage.

In addition, corresponding to the methods provided in the foregoingembodiments, the following case exists: The SAE-TMSI of a UE consists ofan MCC, an MNC, a Pool-id, an MME-id, and an M-TMSI (remaining SAE-TMSIbits). A global MME ID is composed of the MCC+MNC+Pool-id+MME-id. TheSAE-TMSI can uniquely identify a UE globally.

In a UMTS PS domain or GPRS system, the temporary ID of a UE is aP-TMSI. The P-TMSI uniquely identifies a UE in an RA. The P-TMSI and theRAI can uniquely identify a UE globally.

In addition, the following scenario may exist when a 2G/3G network andan SAE network are deployed: a standalone SGSN and a standalone MMEexist, and a combined MME/SGSN also exists. When a UE accesses astandalone SGSN, a P-TMSI and an RAI are allocated to the UE, and aP-TMSI Signature may also be allocated to the UE. The following RAI andP-TMSI combination includes the P-TMSI Signature. When the UE accesses astandalone MME, an SAE-TMSI is allocated to the UE. When the UE accessesa combined MME/SGSN, the combined node may allocate an SAE-TMSI or anRAI/P-TMSI combination to the UE. The SAE-TMSI and the RAI/P-TMSI can bemutually mapped; for example, the MCC+MNC+Pool-id+MME-id in the SAE-TMSIand the RAI can be mutually mapped, and the M-TMSI in the SAE-TMSI andthe P-TMSI can be mutually mapped, or any other mapping method may beused (Certain bits are mapped to the P-TMSI Signature). If the UEaccesses a 2G/3G network, the RAI and the P-TMSI are allocated to theUE. If the UE accesses an LTE network, a combined node allocates anSAE-TMSI to the UE. Thus, when the UE accesses an SAE network, the UEuses the SAE-TMSI (if available) for access. An ENB finds the old MMEthrough the MME-id information in an RRC message or selects a new MME(if the MME corresponding to the MME-id does not exist). If a new MME isaccessed, the new MME may find the old MME and the UE context accordingto the SAE-TMSI, thus obtaining the context of the UE. If the UE hasonly the RAI/P-TMSI and accesses the SAE network, the RAI/P-TMSI ismapped to an SAE-TMSI for access. That is, when the UE accesses the SAEnetwork, the network considers that the ID carried by the UE is theSAE-TMSI, which is the same as the SAE-TMSI that is actually carried bythe UE. Specifically, the UE carries the mapped SAE-TMSI in an RRCConnection Complete message; the ENB finds the old MME (such as acombined node) according to the mapped MME-id or selects a new MME; thenew MME finds the old SGSN and the UE context according to the mappedSAE-TMSI, thus obtaining the context of the UE. When the UE accesses the2G/3G network, the UE uses a P-TMSI and an RAI (if available) foraccess. An RNC finds the old SGSN according to the NRI in the P-TMSIinformation of an RRC message or selects a new SGSN (if the SGSNcorresponding to the NRI does not exist). If the UE has only an SAE-TMSIand accesses the 2G/3G network, the UE accesses the network by mappingthe SAE-TMSI to the RAI/P-TMSI. The foregoing mapping method may be usedto access the old SGSN (a combined node) or a new SGSN. The new SGSNfinds the old MME and the UE according to the mapped RAI/P-TMSI toobtain the context of the UE.

In the foregoing method, the combined node may allocate two IDs (theSAE-TMSI and the RAI/P-TMSI combination) to the UE only when the Idlemode Signaling Reduction (ISR) feature is enabled. Thus, the UE can knowwhether the ISR feature is enabled according to one or two IDs receivedrather than extra signaling or an extra parameter.

Furthermore, there is another solution: when the UE accesses astandalone SGSN, a P-TMSI and an RAI are allocated; when the UE accessesa standalone MME, an SAE-TMSI is allocated; when the UE accesses acombined MME/SGSN, the combined node allocates an SAE-TMSI and anRAI/P-TMSI combination, which cannot be mutually mapped, to the UE. Theproblem is as follows: when a UE supports only one RAT, for example, theUE belongs to the old network and supports only the 2G/3G accesstechnology, and when the ID of an RAT that is not supported is allocatedto the UE, the UE cannot use or even cannot save the ID. Therefore, whenthe UE accesses a network, it may carry the RAT capability supported bythe UE. A combined node obtains the RAT capability supported by the UE;for example, it may obtain the RAT capability from the UE or the contextof the UE to determine how to allocate an ID. For example, the combinednode allocates an RAI/P-TMSI combination to a UE that supports only the2G/3G access technology, allocates an SAE-TMSI to a UE that supportsonly the LTE access technology, and allocates an SAE-TMSI and anRAI/P-TMSI combination to a UE that supports the 2G/3G access technologyand the LTE access technology.

According to the foregoing descriptions, when the UE accesses an MMEthrough the SAE-TMSI, the objective of the invention may be achievedthrough at least the following steps: setting the NRI of the P-TMSIinformation in the RRC message as the MME-id; and setting the MMEaddress information to the old RAI and/or P-TMSI of the NAS message sothat other information may occupy the old RAI, and/or P-TMSI, and/orP-TMSI Signature of the NAS message. It should be noted that thesettings may not be sequential settings or unique settings. For example,certain bits of the Pool-id can be set to the old RAI and certain bitsof the Pool-id can be set to the P-TMSI. In addition, the MME-id may beset both in the old RAI and in the NRI, and the information in the twopositions can be the same or duplicate.

In the foregoing embodiments of the invention, when the MME informationfor uniquely identifying an MME is added to the access message sent tothe old network, the old network can determine and find the MME that isaccessed by the UE in the evolved network without changing the RNC andthe SGSN on the old network. Each network node that communicates withthe UE, such as an MME and an SGSN, may be described as the network.

An embodiment of the invention also proposes a method for providing aTemporary Logical Link Identity (TLLI) when a UE accesses a 2G network:The network receives an access message sent by the UE. The accessmessage contains two TLLIs, namely, the first TLLI and the second TLLI.When the first TLLI information carries the TMSI information (if the RATadopted for the last access is 2G/3G, the TLLI is formed by removing thefirst two bits of the P-TMSI; if the RAT adopted for the last access isSAE, the TLLI is formed by removing the first two bits of the M-TMSI) ofthe RAT of the network that is accessed by the UE at the last time, theRAT is called the first RAT. When the second TLLI information carriesthe TMSI information (if the RAT adopted by the current access is 2G/3G,the TLLI is formed by removing the first two bits of the P-TMSI; if theRAT adopted by the current access is SAE, the TLLI is formed by removingthe first two bits of the M-TMSI) of the RAT of the network that isaccessed by the UE currently, the RAT is called the second RAT. If thetwo RATs are the same, the UE may carry only the information of oneTLLI. If the RAT adopted for the last access is SAE, the old RAI in anNAS message sent by the UE may also be mapped by other information ofthe SAE-TMSI except the M-TMSI. In addition, the NAS message sent by theUE further carries the old RAI of the RAT or the remaining informationof the SAE-TMSI except the M-TMSI. When the two RATs are the same, theNAS message sent by the UE may not carry the old RAI of the RAT or theremaining information of the SAE-TMSI except the M-TMSI.

For example, when the UE moves from an MME of an evolved network to a 2Gnetwork, the first TLLI is formed by removing the first two bits of anM-TMSI and is placed at the normal TLLI position, and the second TLLI isformed by removing the first two bits of a P-TMSI on the 2G network andis placed at a new position (The usage is different from the usage inRelease 7). Other information of the SAE-TMSI except the M-TMSI carriedin the NAS message sent by the UE is mapped to the normal old RAIposition and the old RAI information of the 2G network carried in theNAS message is placed at a new position.

When the UE moves from a 2G network to another 2G network, or when thelatest context of the UE is on the 2G network, the first TLLI is formedby removing the first two bits of a P-TMSI and is placed at the normalTLLI position, and the second TLLI is formed by removing the first twobits of the P-TMSI and is placed at a new position (The usage isdifferent from the usage in Release 7). The UE may also carry only oneTLLI.

If an RAN node is connected to a 2G CN of a later version (such asRelease 8), the RAN node may use the second TLLI to select a CN node. Ifonly one TLLI is available, the RAN node selects a CN node according tothe TLLI. If an RAN node is connected to a 2G CN of an earlier version(such as Release 7 or earlier versions), the RAN node selects a CN nodeaccording to the first TLLI.

The following provides the details.

When a UE accesses an SGSN of a network earlier than 3GPP R8 (Pre-R8),the UE may carry the ID allocated for the RAT adopted for the lastaccess, in the TLLI. For example, the ID allocated for the RAT of a2G/3G network that is last accessed by the UE is a P-TMSI. When the UEaccesses an SGSN of Pre-R8, the UE removes the first two bits of aP-TMSI and places the P-TMSI to the TLLI. According to the receivedTLLI, the SGSN generate a P-TMSI of the NAS. For example, the SGSN addstwo bits whose values are 1 before the TLLI.

If the last accessed MME of the UE is on an evolved network, the UE mayremove the first two bits of the M-TMSI in the SAE-TMSI that isallocated to the UE by the MME and places the M-TMSI to the TLLI, andplaces other bits in the SAE-TMSI to the old RAI.

When the UE accesses a 2G network, the UE provides the information oftwo TLLIs if the UE has two TMSIs. The first TLLI carries the latestTMSI information of the UE, such as the TMSI information used when theUE accesses the network at the last time. The second TLLI carries theTMSI information that supports the same RAT when the UE accesses thenetwork currently. If the UE finds that the two TMSIs are the same, theUE provides the information of only one TMSI. When an RAN node isconnected to a new system, such as an SGSN of 3GPP R8, the access nodeselects a CN node according to only the second TLLI carrying the TMSIinformation that supports the same RAT when the UE accesses the networkcurrently. If only one TLLI is available, the access node selects a CNnode according to the TLLI. If the RAN node is connected to an oldsystem, such as an SGSN of Pre-R8, the RAN node selects a CN nodeaccording to the first TLLI. With the embodiment, the RAN node canselect a proper CN node according to the feature of the connected CNnode. Thus, the old CN node can obtain the latest context informationwithout being upgraded.

Through the foregoing descriptions, it is understandable to thoseskilled in the art that the embodiments of the invention may beimplemented through software and a general hardware platform or throughhardware only. In accordance with such understanding, the technicalsolution of the invention or contributions to the prior art can beembodied by software products. The software products are stored in astorage medium and incorporate several instructions to instruct acomputer device, for example, a personal computer, a server, or anetwork device, to execute the steps provided by each embodiment of theinvention.

Although the invention has been described through above embodiments, theinvention is not limited to such embodiments. It is apparent that thoseskilled in the art can make various modifications and variations to theinvention without departing from the spirit and scope of the invention.The invention is intended to cover the modifications and variationsprovided that they fall in the scope of protection defined by thefollowing claims or their equivalents.

1. A method for accessing a 2G/3G network comprising: obtaining, by aUser Equipment (UE), a temporary identity (ID) allocated by a MobilityManagement Entity (MME) in an evolved network, wherein the temporary IDcomprises MME information for identifying the MME; adding, by the UE,the MME information from the temporary ID to a first P-Temporary MobileStation Identity (P-TMSI) in an access message; sending, by the UE, theaccess message to a Radio Access Network (RAN) node in the 2G/3Gnetwork.
 2. The method of claim 1, wherein the MME information comprisesan MME-Identity (MME-id), the adding by the UE, the MME information fromthe temporary ID, to the first P-TMSI in the access message comprises:setting, by the UE, the Network Resource Identifier (NRI) of the firstP-TMSI in the access message as the MME-id.
 3. The method of claim 2,wherein the sending comprises: sending, by the UE, the access message tothe RAN node to enable the RAN node to select a Serving GPRS SupportNode (SGSN), in the 2G/3G network, whose NRI matches with the NRI of thefirst P-TMSI in the access message.
 4. The method of claim 3, whereinthe SGSN is a combined MME/SGSN, and a NRI of the combined MME/SGSN isset as same as an MME-id of the combined MME/SGSN.
 5. The method ofclaim 3, wherein the method further comprises: adding, by the UE, theMME information from the temporary ID, to a Non Access Stratum (NAS)message carried in the access message.
 6. The method of claim 5, whereinthe NAS message is forwarded by the RAN node to the selected SGSN toenable the SGSN to determine the MME according to the MME informationadded to the NAS message and to request the UE information from the MME.7. The method of claim 5, wherein the adding, by the UE, the MMEinformation from the temporary ID, to the Non Access Stratum (NAS)message carried in the access message comprises: adding, by the UE, theMME information to a second P-TMSI and a Routing Area Identity (RAI) ofthe NAS message.
 8. The method of claim 7, wherein the MME informationcomprises any one of: an MME-id; the MME-id and a Pool-id of the MME;and the MME-id, the Pool-id of the MME and a PLMN-id of the MME.
 9. Themethod of claim 7, wherein the MME information comprises a PLMN-id ofthe MME, a Pool-id of the MME and an MME-id, the adding the MMEinformation to the second P-TMSI and the RAI of the NAS messagecomprises: setting a NRI of the second P-TMSI as the MME-id; and addingthe PLMN-id and the Pool-id to the RAI.
 10. The method of claim 7,wherein the method further comprises: adding, by the UE, information ofthe temporary ID other than the MME information to any one orcombination of the second P-TMSI, the RAI or a P-TMSI Signature in theNAS message.
 11. The method of claim 1, wherein the access message is aRadio Resource Control (RRC) message.
 12. A user equipment (UE),comprising: a receiver, a transmitter and a Mobility Management Entity(MME) information adding module, wherein the receiver is configured toobtain a temporary identity (ID) allocated by an MME in an evolvednetwork, wherein the temporary ID comprises MME information foridentifying the MME; the MME information adding module is configured toadd the MME information from the temporary ID to a first P-TemporaryMobile Station Identity (P-TMSI) in an access message, when the UEaccessing a 2G/3G network; and the transmitter is configured to send theaccess message to a Radio Access Network (RAN) node in the 2G/3Gnetwork.
 13. The UE of claim 12, wherein the MME information comprisesan MME-Identity (MME-id), the MME information adding module is furtherconfigured to set the Network Resource Identifier (NRI) of the firstP-TMSI in the access message as the MME-id.
 14. The UE of claim 13,wherein the transmitter is further configured to send the access messageto the RAN node to enable the RAN node to select a Serving GPRS SupportNode (SGSN), in the 2G/3G network, whose NRI matches with the NRI of thefirst P-TMSI in the access message.
 15. The UE of claim 14, wherein theSGSN is a combined MME/SGSN, and a NRI of the combined MME/SGSN is setas same as an MME-id of the combined MME/SGSN.
 16. The UE of claim 12,wherein the MME information adding module is further configured to addthe MME information from the temporary ID, to a Non Access Stratum (NAS)message carried in the access message.
 17. The UE of claim 16, whereinthe MME information adding module is further configured to add the MMEinformation to a second P-TMSI and a Routing Area Identity (RAI) of theNAS message.
 18. The UE of claim 17, wherein the MME informationcomprises any one of: an MME-id; the MME-id and a Pool-id of the MME;and the MME-id, the Pool-id of the MME and a PLMN-id of the MME.
 19. TheUE of claim 17, wherein the MME information comprises a PLMN-id of theMME, a Pool-id of the MME and an MME-id, and the MME information addingmodule is further configured to set a NRI of the second P-TMSI as theMME-id, and to add the PLMN-id and the Pool-id to the RAI.
 20. The UE ofclaim 15, wherein the MME information adding module is furtherconfigured to add information of the temporary ID other than the MMEinformation to any one or combination of the second P-TMSI, the RAI anda P-TMSI Signature in the NAS message.
 21. A Radio Access Network (RAN)node in a 2G/3G network, comprising: a receiver, configured to receivean access message sent by a User Equipment (UE) that accesses the 2G/3Gnetwork, wherein the access message comprises Mobility Management Entity(MME) information identifying an MME accessed by the UE in an evolvednetwork, and the MME information is added by the UE from a temporary IDallocated by the MME to a first P-Temporary Mobile Station Identity(P-TMSI) in the access message; and a processor, configured to select acorresponding Serving GPRS Support Node (SGSN) in the 2G/3G network forthe UE according to the first P-TMSI in the access message.
 22. The RANnode of claim 21, wherein the MME information comprises an MME-Identity(MME-id), and the Network Resource Identifier (NRI) of the first P-TMSIin the access message is set as the MME-id; wherein the processor isfurther configured to select a SGSN in the 2G/3G network, whose NRImatches with the NRI of the first P-TMSI.
 23. The method of claim 22,wherein the SGSN is a combined MME/SGSN, and a NRI of the combinedMME/SGSN is set as same as an MME-id of the combined MME/SGSN.